Past-month marijuana use in young adults (YAs; ages 18-24) is nearly 3 times higher than the national average for adults (19% vs 7%, respectively).1 High rates of the co-use of marijuana and tobacco (MJ-TOB)2-8 and lower perceptions of risk of using marijuana have been found in this age group.1 Co-use of MJ-TOB is correlated with greater marijuana and nicotine dependence,9,10 poor tobacco cessation and marijuana outcomes,11,12 increased cancer risk,12-15 alcohol and other drug use,2,3,16 anxiety and depression,17-20 and poor academic achievement.21,22 The rising use of alternative tobacco products [hookah, e-cigarettes, little cigars/cigarillos (LCCs)] in YAs is particularly concerning, as they can be used as vehicles to consume marijuana,10,15,23-26 and further expand the nicotine/tobacco market for this age group. Combined use of marijuana within a tobacco product (i.e., within a blunt) may be particularly appealing because of the intense high that extends beyond use of either product alone or on separate occasions. This may temper in the moment risk perceptions of marijuana and encourage use of other substances, placing users at greater risk for drug related consequences. The Benefits/Risk Perception model29, 30 indicates that perceived risk for marijuana is an important correlate and predictor of marijuana use and future substance use outcomes. However, little data exist on how in the moment risk perceptions of marijuana vary by contextual patterns of MJ-TOB use, and how these perceptions and patterns of use dynamically change over time. This R21 will address these gaps in the research by examining the impact of different patterns of MJ-TOB co-use and daily marijuana risk perceptions on substance use and mental health outcomes at 6-month follow-up in young people. A sample of 144 YAs who are current tobacco users and consume marijuana > 2 a week2 will track alcohol, tobacco, marijuana, and other drug use, and in the moment thoughts, emotions, social contexts, and marijuana risk perceptions for 28 days using ecological momentary assessment (EMA) and will be re-assessed 6 months later. We propose to: (a) characterize different patterns of MJ-TOB co-use as a function of proximity in use; (b) examine the main and interactive effects of MJ-TOB patterns of co-use and daily/momentary marijuana risk perceptions on changes in, or progression of substance use and mental health outcomes (depression, anxiety) at a 6-month follow-up; and (c) explore how immediate social and emotional contexts (measured via EMA), availability of marijuana, and psycho-social factors impact daily marijuana risk perceptions and differ across patterns of MJ-TOB co-use.